The transport of macromolecules through the nuclear envelope is a complex process that involves many protein factors and an energy source such as GTP (see E. Izaurralde and I.W. Mattaj, Cell 81:153, 1995 and D. Gorlich, et al., Science 271:1513, 1996 for review). Besides the many components of the nuclear pore complex (NPC), several soluble proteins localized to one or both sides of the nuclear envelope are also essential. Key to this process are the nucleotide GTP, a GTPase (Ran) and the nuclear GTP:GDP exchange factor RCC1. We showed previously that RCC1, which is required for export of some messenger RNAs (mRNAs), is also required for export of precursors of small nuclear RNAs (pre-snRNAs) and ribosomal RNAs (rRNAs) but not for that of transfer RNAs (tRNAs). Because pre-snRNAs undergo maturation in the cytoplasm, this inhibition resulted in accumulation of immature RNAs in nuclei.
Keene and coworkers had previously shown that infection of cells by the cytoplasmically replicating negative strand vesicular stomatitis virus (VSV) resulted in inhibition of processing of pre-snRNAs. The similarities between inactivation of the RCC1-dependent export pathway and VSV infection, with respect to accumulation of unprocessed pre-snRNAs, led us to ask if VSV infection might inhibit export of these RNAs from the nuclei of infected cells.
Two VSV gene products have been reported to be present in the nucleus after infection. The two gene products are the newly synthesized 47 nucleotide leader RNA and a pre-existing component of infecting virions, a .about.30 kDa protein called the matrix protein (M protein). Since transcription, but not protein synthesis, is required for inhibition of pre-snRNA maturation, our first attempts were directed at answering the question of whether the leader RNA was responsible for this inhibition. Our inability to reproduce the inhibition by VSV leader RNA led us to question the role of M protein. This protein, present in about 1800 copies per virion, may be released from virions only upon transcription of the leader RNA, at which point it would e free to migrate into the nucleus.